Preheat oven to 350 degrees. Lightly grease a 9" X 13" glass baking dish.
Wash and cut up squash and onion into pan with enough water to cook until fork tender. While squash is cooking scramble fry sausage and drain off grease. When squash is done, drain,add stick of butter, and mash with a potato masher.
To this mixture add cream of celery soup, beaten eggs, sausage, cheese and stir until mixed well. Pour into glass baking dish. Bake for approximately 40 to 45 minutes or until lightly browned and set.